In UTRA (Universal Mobile Telecommunication System Terrestrial Radio Access) TDD mode within 3GPP, both dedicated and shared channels are supported. Dedicated channels give a reserved resource to a user, in contrast to shared channels in which a pooled resource is dynamically shared amongst users. Dedicated channels are best suited to near constant bandwidth traffic, typically speech or streamed audio/video. Shared channels are more suited to bursty traffic, typically packet data services such as internet traffic.
In UTRA TDD mode, when a dedicated channel (DCH) is admitted, physical resources (channelisation codes and timeslots) are reserved for its exclusive use. This reservation is often called channel allocation. Two forms of channel allocation are employed:                fixed channel allocation (FCA)        dynamic channel allocation (DCA).        
In FCA, the codes and timeslots for the DCH are selected at random from those available in the cell.
In DCA, the code and timeslot assignment is intelligent. The following classification of DCA schemes is known, for example from the publication by H. Haas and S. McLaughlin “A dynamic channel assignment algorithm for a Hybrid TDMA/CDMA-TDD interface using a novel TS-opposing technique” in IEEE J. Sel. Areas Comms. 19(10) 2001:                Traffic-adaptive channel allocation: based upon the traffic loading on neighbouring cells, a common pool of channels (codes and timeslots) is shared such that heavily loaded cells receive more channels. Adjusting the downlink-uplink timeslot division in a cell(s) may also be considered here. (note that if the division is not equal in every cell then UE-UE (remote station-remote station) and Node B-Node B (base station-base station) interference sources arise).        Re-use partitioning: this may be employed when an operator has a number of carriers which may be deployed in a re-use pattern.        Interference-based DCA: channels are assigned based upon interference power measurements at the Node B or the UE. These schemes match well to TD-CDMA which is typically interference limited, and they may be deployed in a decentralised architecture giving low algorithmic complexity.        
DCA schemes may be either centralised or decentralised. A centralised algorithm would be located at the RNC (Radio Network Controller) and would exploit measurements made at a number of Node B's and UEs in its decision making. The impact of an admission in one cell on the performance of admitted connections in the same and other cells may be determined. Resources from a number of cells may be considered on a collective or pooled basis. Such an algorithm requires significant signalling, may be computationally complex, and may not scale as the number of cells under the RNC is increased.
A decentralised algorithm manages DCH admissions for a single cell (typically). It is located at the RNC, where admission is performed, and where measurements are available. The complexity is considerably reduced, and scaling considerations are easy to estimate (the total complexity for the RNC is proportional to the number of cells under the RNC).
DCA may also be used to reassign the resources of calls in progress to permit the admission of new calls, or to improve their QoS. For example, a voice user may be experiencing high interference in one timeslot, and the DCA algorithm may reassign the user to another timeslot which has lower interference levels.
Call Admission Control
DCA works very closely with Call Admission Control (CAC). Execution of a CAC algorithm is required when UEs are supported with some guarantees of quality of service. Users of 3GPP traffic class=“Interactive” only expect to receive better service than other interactive users with lesser traffic handling priority, whilst users of 3GPP traffic class=“background” have no expectations (this is truly best effort). However, users of 3GPP traffic class=“conversational” or “streaming” have delay and bandwidth requirements. The CAC needs to balance the conflicting requirements of low blocking probability and low dropping probability. Dropping occurs when an ongoing call is prematurely terminated and is perceived by users as more objectionable than blocking (when the system does not allow a call to be initiated in the first place): the blocking probability is thus set higher than the dropping probability. The CAC maintains the loading on the network below a threshold level such that the dropping probability is acceptable.
Network loading may be measured in terms of:                number of UEs admitted (of each traffic class/quality of service parameters)        Node B interference (uplink)        Node B transmit power (downlink)        
Interference is composed of two parts, intracell interference arising from transmissions to/from UEs attached to the same cell, and intercell interference arising from transmissions to/from UEs attached to other cells.
CAC is needed:                on new call attempts        on handovers        on channel reallocation initiated by DCA.        
In the publication “D06: Conceptual studies on Radio Resource and Qos Management Algorithms” (available at web address http://www.arrows-ist.upc.es/publications/deliverables/Summary_Arrows-D06.pdf), a distinction is drawn between DCA and CAC. A first DCA algorithm described in this application maintains an ordered list of timeslots which should be considered when an admission is to be made (this could be a new call, a handover, or a reallocation). The CAC takes a timeslot from the top of this list and evaluates whether the addition of the UE into this timeslot would generate acceptable interference to existing calls. Additionally, a second DCA algorithm can identify calls for reallocation from one timeslot to another—for example, to improve speech quality. However, this publication does not enable dedicated and shared channels to coexist.
Coexistence of Dedicated and Shared Channels
Although the coexistence of dedicated channels (managed by DCA) and shared channels (managed by a radio scheduler) has not been addressed for 3GPP TDD, in the past, to the knowledge of the inventors hereof, clearly there are two possible methods of assigning the codes and timeslots to dedicated and shared channels:                segregation: slots are used exclusively by either dedicated or shared channels        mixing: slots support both dedicated and shared channels at the same time.        
A discussion of a possible interference based DCA technique is included in 3GPP specification TR 25.922 (available from the website www.3gpp.org). The method is based upon interference measurements made by the UE and the Node B (timeslot Interference Signal Code Power or ISCP, i.e., intercell interference). The algorithm is decentralised and located at the RNC. The algorithm covers both slow DCA and fast DCA. Slow DCA involves adjustments to the DL/UL (downlink/uplink) split across cells. The DL/UL split is adjusted for each cell independently of other cells. Fast DCA allocates resources more rapidly. Code pooling and timeslot pooling are discussed (in code pooling a DCH is allocated a number of codes in the same timeslot, whilst in timeslot pooling a single code and multiple timeslots are used). Channel reallocation (intra-cell handover) can be triggered to cope with varying interference conditions, or to reduce fragmentation of codes and timeslots that a DCH uses. UE measurements to support DCA include ISCP, path loss measurements, link quality measurement and UE transmit power values.
In the publication “D06: Conceptual studies on Radio Resource and Qos Management Algorithms” referred to above and in the publication by M. Haardt et al., “The TD-CDMA based UTRA TDD Mode”, IEEE J Sel Areas Comms 18(18), August 2000, the DCA algorithm generates a priority list of timeslots according to long and short term recording and statistical evaluation of interference, at the UE and the Node B. This is used by the CAC. The DCA algorithm described reallocates resources to minimise the number of timeslots used. However, these publications do not enable dedicated and shared channels to coexist.
In the publications by Berg, “Maintaining high capacity for centralised DCA with limited measurements and signalling”, PIMRC 1999, and “Radio resource management in bunched personal communication systems”, PhD Thesis, March 2002, Royal Institute of Technology, Stockholm, a centralised DCA algorithm has been evaluated that exploits knowledge of the gain matrix (the matrix of path gains between UEs and Node B's) and the transmit power of each UE. The method ensures that all SIR targets are met, or the new call is not admitted. The first of these publications suggests ways to compensate for gaps in the gain matrix—by using values taken by other UEs in the same cell, or by setting a higher SIR target for the new admission than necessary and allowing the power control to adjust this during the call. A third method discards the gain matrix approach and instead admits the UE to the timeslot in which it measures the minimum interference. However, such gain matrix calculations are complex.
In the publication by I. Forkel et al., “Dynamic channel allocation in UMTS Terrestial Radio Access TDD systems”, VTC 2001, the admission attempt is made into the DL and UL timeslots with the minimum interference. The admission is allowed if the interference levels in the two directions are below their respective threshold values (these can be service type dependent). A more advanced scheme allows the allocation for a voice call to be changed if the bit error rate (BER) exceeds a threshold for a given duration. The DCA scheme gives C/I gains over FCA but little voice capacity improvement. Furthermore, this publication does not enable dedicated and shared channels to coexist, and the DCA and CAC metrics are crude.
In the publication by I. Forkel & T. Kriengchaiyapruk, “Management of circuit and packet switched data in UMTS terrestrial radio access networks”, 3G Wireless 2001, a ‘timeslot scoring method’ is applied where a timeslot is chosen at random from the set of timeslots whose interference falls below a threshold, and have sufficient capacity. In this technique, code pooling is used, otherwise the DCA and the CAC would be more complex and less reliable. However, this publication does not enable dedicated and shared channels to coexist.
From patent publication EP0817521, “Interference based dynamic channel assignment”, it is known to use long and short term interference measurement lists on DL and UL. The algorithms are decentralised, on a per cell or per sector basis.
From patent publication EP0986928, “DCA method in a cellular radio communication network”, it is known to maintain priority indices based upon periodic measurements of radio parameters. This publication is orientated to a frequency channel assignment. However, this publication does not enable dedicated and shared channels to coexist and its application to timeslot assignment is not clear.
In patent publication EP1063791, “CDMA communication method using a dynamic channel code assignment, and a base station performing the method”, a channelisation code used by a UE is changed in response to interference measurements. However, this publication is restricted to a limited algorithm for an FDD (Frequency Division Duplex) system.
There are known a number of publications involving Call Admission Control for WCDMA, but these are generally oriented towards 3GPP FDD (Frequency Division Duplex) mode. In the publication by H. Holma & A. Toskala (editors), “W-CDMA for UMTS”, John Wiley, 2000, an interference based algorithm is described for the FDD mode. On the uplink, the expected interference at the Node B after admission is compared to a threshold. The difficulty is in estimating how much additional interference is generated by the new admission. Two solutions are described, the derivative and integral methods which both exploit knowledge of the shape of the interference versus load curve I=1/(1−η) where η is the load). On the downlink the expected transmit power following admission is compared to a threshold. The increment in power is estimated by an open loop calculation. The downlink algorithm is applicable to the TDD mode. The uplink algorithm is not applicable to TDD mode since in TDD mode, the Node B detector eliminates most of the intracell interference (thus adding another user to a TDD cell will not increase the detected Node B interference in that cell).
In the publication by J. Lee & Y. Han, “Downlink admission control for multimedia services in WCDMA”, IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC) 2002, a simple downlink method is described that uses a transmit power threshold value.
There are a number of papers where uplink CAC is based on Node B interference estimation. They differ in the method in which the interference increment is calculated, and also in whether the impact on the serving cell or on neighbouring cells is included too. The publication by Kim et al., “SIR-based call admission control by intercell interference prediction for DS-CDMA systems”, IEEE Comms. Letters, 4(1), 2000, extends the work of Z. Liu and M. E. Zarki in the publication “SIR-based call admission control for DS-CDMA systems”, IEEE J. Sel. Areas Comms., 12, 1994 to base an algorithm on ‘Residual Capacity’, which expresses the number of calls which can be accepted in each cell following the admission of the user in question. The residual capacity is calculated for the intended serving cell and all the neighbours for which the UE is able to make pilot/beacon measurements of. If the residual capacity is greater than or equal to 1 for all cells, then the call is admitted. This method may be useful if the SIR needs of the UEs are the same and sufficient measurements are available to the algorithm. The complexity is moderate. However, this method is limited by the restriction of equal SIR needs and inappropriately includes intracell interference in its workings.
In the publication by N. Dimitrou & R. Tafazolli, “Quality of service for multimedia CDMA”, IEEE Comms. Mag. July 2000, there is presented a simple uplink CAC algorithm based on an interference threshold, which can consider the impact on the local cell or on multiple cells. However, this algorithm is relatively inaccurate and unsophisticated.
In the publication by F. Gunnarsson et al., “Uplink admission control in WCDMA based on relative load estimates”, International conference on comms. 2002, a formula is derived for the relative (uplink) load on a cell as a function of the SIR target of the service, the path gain between each UE and its serving site, and the path gain between the UE and the cell in question. The CAC algorithm calculates the new relative load in each cell which would follow an admission, and compares these values against a threshold. The relative load in cell j is:
            L      ^        j    =            ∑              i        =        1            M        ⁢                  ⁢                            CTIR          i                ⁢                  g          ij                            g        ik                            where        i is the mobile number, ranging from 1 to M, in the system,        CTIRi is the target value of the carrier to total interference ratio for mobile i at its serving site, k,        gij is the path gain from mobile i to cell j,        gik is the path gain from mobile i to its serving site, cell k.        
A call is admitted provided the relative load is less than the threshold for each cell considered. The authors claim that measurements of path gain are more accurate than those of noise or interference at the Node B (as used in the publication by Kim et al., “SIR-based call admission control by intercell interference prediction for DS-CDMA systems”, referred to above). If a mobile is too distant to be able to measure the beacon of a cell it is not included in the relative load calculation for that cell. Measurement load is minimised by relying upon initial measurements and thereafter those available at handover. Cell centre UEs offer few measurements (no handovers) but their interference contribution is less than those of cell edge UEs. The approach may be used for multiple services (the CTIR value is changed appropriately). This paper considers FDD where intracell interference cannot be cancelled and is not directly applicable to TDD for the serving cell.
Thus, although many different algorithms are known for channel allocation, these are not optimal for all conditions and systems. In particular, known channel allocation schemes are not ideally suited for communication systems comprising both shared and dedicated communication channels.
Hence, an improved system would be advantageous and in particular a system allowing increased flexibility, improved performance, improved utilisation of shared and dedicated channels and/or improved suitability of a range of communication systems including TDD communication systems would be advantageous.